Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19.
The gas turbine engine 10 operates in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts 26, 28, 30.
In view of the above, it will be appreciated that a number of blade assemblies are provided in compressor and turbine stages of a gas turbine engine. These blade assemblies may disintegrate for a number of reasons such as fatigue, impacts and mechanical failures. In such circumstances, parts of the blade assembly will impact a containment casing. In order to ensure that, particularly with regard to gas turbine engines utilised in aircraft, such fragments do not exit the engine, it is necessary to include a casing, which is sufficiently strong to prevent fragment escape. In such circumstances, consideration is made with regard to energy transferred by blade fragments to the casing during impacts. Reductions in casing thickness will reduce significantly the weight of the casing and therefore increase its acceptability with regard to use in an aircraft.
In view of the above, it has been known to introduce lines of weakness within blade arrangements in order to precipitate, when appropriate, fragmentation of blade segments into manageable sizes with reduced impact energy and therefore impact penetration potential with regard to casings. Such lines of weakness are drilled or machined or moulded into the blade assembly whether it be of a metal or composite form in order to define appropriate break lines. Unfortunately, introducing such lines of weakness in the form of grooves will create moisture paths, which may precipitate cracking. Such problems are further exacerbated if to reduce blade assembly weight hollow sections are created into which moisture can enter and cause “freeze thaw” cycling and again cracking which may result in premature failure of the blade arrangement.
Machining to introduce lines of weakness can also result in latter stage tool breakage and so rejection of a blade assembly. Furthermore a hollow cavity root within a blade can cause stress concentration in an already relatively high stressed region of a blade assembly. Furthermore, the radial extent of the cavity is limited by manufacturing processes available. The above problems with regard to blade disintegration are exacerbated with regard to blisk (bladed disk) blade assemblies having blades secured to a disc, bling (bladed ring) blade assemblies where a blade is secured to a ring and blum (bladed drum) blade assemblies where a blade is secured to a drum. These problems occur whether the blade assembly is made from a metal or composite.
The disadvantage of a conventional blisk or bling or blum blade arrangement is that the failure can lead to a further failure of the whole and complete structure. If the whole blade arrangement fails then segments released can be of relatively high energy and therefore cause difficulties with regard to containment within a casing.